1. Field of the Invention
This invention relates to tensile testing apparatuses and more particularly, tensile testing apparatuses which simulate dynamic conditions.
2. Description of the Prior Art
Tensile testers are known in the prior art which can test and evaluate the tensile strength of a sample of sheet material or fiber under static conditions. An example of such a prior art device is sold by the assignee herein under the tradename xe2x80x9cMICRO 350 UNIVERSAL TESTER.xe2x80x9d This apparatus is provided with two jaws, one being stationary and the second being movable, to grip opposing ends of a sample. The sample is engaged at opposing ends, with the entire assembly of the jaws and sample being stationary at the onset of a test. In conducting the test, the movable jaw moves away from the stationary jaw until the sample fails. During the course of the test, measurements of stress and strain are made and the tensile characteristics of the sample are determined.
It has been determined that tensile properties of certain materials, such as paper, change when placed under dynamic conditions. In particular, it has been found that the modulus of elasticity of certain materials varies with the sample travelling at various velocities. Consequently, the elasticity/brittleness of the sample can be considered a function of the velocity the sample is travelling when placed under tensile load. For example, a sheet of paper passing through a photocopying machine is forced to pass over a fusing roll which heats the sheet and causes curling thereof. To remove the curl, the sheet is quickly passed over a de-curling bar. While passing over the de-curling bar, the sheet of paper is subjected to tensile loading. It has been uncovered that with a sheet of paper moving, i.e. being under dynamic conditions, the tensile characteristics of the sheet of paper vary from that measured with the static procedure described above. As can be readily appreciated, the determination of dynamic tensile properties of a sample would allow a designer to properly take account of changing tensile properties. For example, the shape, angles of engagement and the rate of engagement of the sheet of paper with the de-curling bar can be properly designed for to allow the sheet to pass as quickly as possible over the de-curling bar without failing. At present, designers often take iterative steps to determine acceptable design criteria of the de-curling procedure.
It is an object of the subject invention to provide an apparatus for testing tensile properties of a sample under dynamic conditions.
The aforementioned object is met by a system for testing the tensile characteristics of a sample under dynamic conditions. In particular, the system includes an apparatus adapted to accelerate the sample to a desired test speed and maintain the test speed through a predetermined test run, and a microprocessor for controlling the apparatus, data collection and data evaluation. Generally, the apparatus includes an elongated housing, a rail mounted to the housing, two jaw assemblies mounted to the rail for translation along the length thereof, and a linear motor for driving one of the jaw assemblies.
The apparatus may be used to test the tensile properties of any sheet material, including paper, foil, plastic, and the like, as well as individual strands, filaments, and threads. Each of the jaw assemblies is formed with a clamping jaw for gripping one end of the sample. The jaw assemblies include a leading jaw assembly and a follower jaw assembly, wherein a detachable coupling is provided for forming a connection therebetween. The linear motor is mechanically connected to the leading jaw assembly. The follower jaw assembly is also driven by the linear motor, due to the connection to the leading jaw assembly formed by the detachable coupling.
Prior to conducting a test, a particular length of the rail is pre-designated as the length of a test run. In conducting a test run as described below, the linear motor must accelerate the jaw assemblies to the desired test speed upon entering the test run. To ensure the test speed has been achieved, the apparatus is configured to reach the test speed prior to entering the test run. At the initial point of the test run, a catch mechanism is provided for instantaneously stopping the follower jaw assembly. It is desired that the catch mechanism completely halt all subsequent movement and rebounding of the follower jaw assembly upon engagement therewith.
Two linear encoders are mounted to the jaw assemblies to allow for observation of various characteristics of the jaw assemblies. In particular, the velocity and location of the leading jaw assembly are monitored, as well as the spacing between the two jaw assemblies. In addition, a load sensor is mounted to one of the clamping jaws to measure the force being applied to the sample. All of the measured data is transmitted to the microprocessor for evaluation in determining the stress-strain characteristics of the test sample. Also, the linear motor is controlled by the microprocessor, and the velocity measurements provide real-time data to continuously determine whether the desired test speed is being maintained. A closed loop between the microprocessor, the linear encoder measuring velocity, and the linear motor can be formed to accordingly adjust and maintain the desired test velocity.
In conducting a test run, a sample is caused to be gripped by both clamping jaws with the jaw assemblies being coupled together. A desired test velocity is inputted into the microprocessor. The use of a linear motor advantageously allows for a wide range of test velocities which may be selected being in the range of 0.05 meter/min to 5 meters/sec. Once the sample is loaded and the test velocity is selected, the test is initiated and the linear motor accelerates the coupled jaw assemblies to the test velocity. With the test velocity having been achieved, the test run is initiated upon the follower jaw assembly engaging the catch mechanism and being caused to be instantaneously stopped. The movement of the leading jaw assembly, however, is unhindered. Consequently, the leading jaw assembly moves away from the follower jaw assembly, thus causing the sample to elongate. Data is acquired by the linear encoders and the load sensor and transmitted to the microprocessor for evaluation. The test is completed upon the leading jaw assembly having translated the full length of the test run. Depending on the elasticity of the sample being tested, the sample may have failed during the course of the test. Acquired data is used to determine the stress-strain characteristics of the sample at the test velocity, from which the modulus of elasticity may be determined. Subsequent tests may be performed on the same material to determine the effects of different velocities on the modulus of elasticity of the sample material being evaluated.
In one embodiment of the invention, the test is initiated without tension being applied to the sample. Alternatively, one or both of the clamping jaws may be formed to be moveable relative to the respective jaw assemblies to allow for pre-tensioning of the sample prior to initiation of the test run. As further modifications, the apparatus can be disposed to have the rail vertically or horizontally aligned.
These and other features of the invention will be better understood through a study of the following detailed description and accompanying drawings.